In recent years, worldwide demand for wireless cellular communications has increased dramatically. Radiotelephones manufactured to meet this burgeoning demand must adhere to standards such as the Global System for Mobile Communications (GSM) standard. Another standard, the Digital Cellular System (DCS) standard, is based on GSM, but is directed towards higher cell density and lower power. A third standard, Personal Communications Services (PCS) is a “catch all” for many digital cellular systems, including GSM, operating in North America. These standards all require precise output power control over a large dynamic range in order to prevent a transmitter located in one cell from interfering with the reception of transmissions from other transmitters in neighboring cells.
A key component common to all radiotelephones is a radio frequency (RF) power amplifier (PA). In modern digital radiotelephones, power amplifiers receive as input a frequency or phase-modulated RF carrier. The radio frequency carrier is what “carries” digital information such as digitized voice or data to a cellular base station. Before reaching the power amplifiers, the RF carrier is too weak to be received by a cellular base station. Therefore, it is the function of the power amplifiers to boost the power of the RF carrier to a level sufficient for reception by a cellular base station.
Some mobile terminals, such as modern digital radiotelephones, also include a cascaded directional couplers circuit. The cascaded directional couplers circuit comprising a first directional coupler connected in series with a second directional coupler is usable to provide a feedback of a portion of the power amplifiers' outputs in order to accurately control the output power of the power amplifiers.
A typical cascaded couplers circuit is well suited for light coupling applications having a coupling that is greater than 30 decibels (dB). However, as coupling increases, the first directional coupler that is typically close to a load such as an antenna appears to be significantly more inductive to an isolated port of the second directional coupler that is connected in series with a coupled port of the first directional coupler via a conductive path such as a metal trace. Consequently, a problem arises in that in applications that require heavy coupling that is less than 22 dB, the directivity of the second directional coupler is degraded.
Thus, there remains a need to provide a type of compensation that will eliminate the degradation of the directivity when heavy coupling is implemented with the cascaded directional couplers circuit.